Firefighting in civilian, particularly commercial, and military aircraft requires equipment which uses halons, also known as halogen hydrocarbons, as fire extinguishing agents. However, the use of such halons is discouraged in view of their presumed adverse effect on the environment. Thus, halon replacement agents are known which have comparable fire extinguishing characteristics, however with a lesser adverse effect regarding the so-called greenhouse effect. In other words, halon replacements have a smaller or no adverse effect on the ozone layer. Halons assume their liquid state under a pressure of about 25 bar (gage) and are stored in suitable containers for holding these fire extinguishing agents, for example in an aircraft. Generally, such containers have an outlet that is normally closed with a frangible closure membrane. A nitrogen cushion is usually provided in the container above the halon in its liquid state. These membranes permit connecting the container to a distribution pipe system by destroying the membrane, for example by igniting a pyrotechnical membrane control system with an electrical spark. As soon as the membrane is destroyed, the halon flows through the pipe system to the enclosed space where a fire has started. Conventional nozzles connected to the discharge end of the pipe system distribute the fire extinguishing agent in the enclosed space. The pyrotechnical closure system is usually remote controlled through an electrical switch in the cockpit. Fire detectors are installed in the enclosed space and provide a warning signal to a control station such as the cockpit so that the release of fire extinguishing agent can be immediately triggered by a crew member or automatically.
Conventionally, the fire extinguishing agent such as halon flows without flow restriction out of a first fire extinguishing agent holding container through the pipe system to the enclosed space until the first container is empty, whereby the pressure in the container now corresponds to the atmospheric pressure or to the pressure in the aircraft cabin or loading space. The continuous discharge of fire extinguishing agent from a first container assures that a high initial concentration of extinguishing agent is provided in the enclosed space leading to a rapid suppression or suffocation of the fire.
Simultaneously with the discharging of extinguishing agent from the first container, or after a complete emptying of the first container, the pyrotechnical closure system of a second container is triggered. The second container is connected to the pipe system through a water adsorption filter and a solid particle filter positioned in a portion of the pipe system leading out of the second container into the discharge pipe system. A pressure reduction throttle is provided in this portion of the pipe system for reducing the pressure of the outflowing extinguishing agent. As a result a relatively small, strongly throttled extinguishing agent mass passes from the extinguishing agent container through the pipe system to the fire location. Such restricted mass flow nevertheless makes sure that in the enclosed space, where a fire has started, there will always be maintained an extinguishing agent concentration, which does not fall below a minimal concentration required for preventing rekindling. For this purpose a diaphragm or control aperture is arranged downstream of the pressure reducer for a precise limitation of the halon throughflow to certainly prevent rekindling. Downstream of the diaphragm or control aperture there is arranged a check valve for preventing a return flow of extinguishing agent out of the pipe system into the second container. This check valve also protects the pressure reducer against a pressure shock occurring when the first container is opened. A relatively small extinguishing agent mass flow is required for suppressing any rekindling of the fire with certainty. Thus, for example a value of the mass flow in the range of 0.05 to 0.5 kg/min is sufficient to avoid rekindling. Due to the high pressure drop of the extinguishing agent downstream of the pressure reducer, the extinguishing agent changes from its liquid phase into its gaseous phase.
It is also known that the fire extinguishing agents contain contaminations in the form of non-volatile materials such as oil, grease, solid particles or the like which have a tendency to accumulate at the location of the phase change, namely preferably in the area of the pressure reducer. This is a disadvantage which becomes worse with time due to the relatively small mass flow of the halon extinguishing agent and due to the low temperature up to −50° C. These conditions lead to an accumulation of contaminations which have a negative influence on the closed loop control characteristic of the pressure reducer which eventually may lead to a total system shut down of the entire firefighting equipment or system.